Pollutants from the exhaust gas of automobiles, especially from diesel engine vehicles, abraded tire dust, asphalt dust, and further, a large volume of pollutants contained in the flue gas from industries accumulate on road surfaces. These pollutants include organic pollutants, such as polycyclic aromatic-hydrocarbons, heavy metals such as chrome and cadmium, nutrients such as nitrogen and phosphorus, which cause eutrophication, and further include materials of potential environmental hormones. These pollutants and the like are washed out by rainfall and pass into storm drains adjacent to paved surfaces, such as streets and parking lots and through curb inlets, and flow into the sewage systems or into the rivers.
Larger spills of oil can also flow into storm drains, such as from loading docks, gas stations, and the like. In addition, quantities of oil or other hydrocarbons are frequently spilled on the ground and subsequent water flow, such as from rain, can cause the oil to flow into storm drains. This is referred to as “non-point-source pollution.”
The volume of oil from non-point-source pollution in typical water runoff is surprisingly large and consequently, cause enormous annual costs, both financial and environmental, by contaminating natural receiving waters.
When the sewage system is a combined flow system, pollutants are treated and disposed of at the sewage treatment plants. But if the pollutants include heavy metals, such as chrome or cadmium, the sludge needs a special treatment.
On the other hand, when the sewage system is a separate flow system, road surface run-off water containing pollutants flows into the rivers with no proper treatment, the pollutants potentially cause a eutrophication or environmental water pollution, and causes a disruption of the ecosystem.
The chemicals, such as insecticides or herbicides, in the effluent from rice fields or the like, and the suspension solids caused by plowing the rice fields for young rice plants flow into the rivers and cause water pollution.